Molecular sieves are an important class of materials used in the chemical industry for processes such as gas stream purification and hydrocarbon conversions. Molecular sieves are porous solids having interconnected pores of different sizes. Molecular sieves typically have a one-, two- or three-dimensional crystalline pore structure that selectively adsorb molecules that can enter the pores and exclude those molecules that are too large.
The pore size, pore shape, interstitial spacing or channels, composition, crystal morphology and structure are a few characteristics of molecular sieves that determine their use in various hydrocarbon adsorption and conversion processes.
During synthesis, the reagents are mixed to form a “gel” that may be aged at a temperature for a given period before reacting for a time to provide a crystalline molecular sieve. Conventional processes used in the synthesis of these materials may employ reactors or autoclaves for step-wise mixing, gel aging and final crystallization of the product. Molecular sieves may also be produced in a continuous process. Due to the costs associated with such crystallizers, it is advantageous to maximize the output of each unit, which conventionally can be accomplished in two ways: minimize crystallization time or maximize yield.
In order to minimize crystallization time, it is customary to monitor the degree of crystallization so that the reaction may be terminated as soon as the product achieves a requisite yield. Conventionally, the termination point is determined by withdrawing a sample of the reaction mixture and measuring its crystallinity by powder X-ray diffraction (XRD) of a dried sample. This is relatively intensive in terms of its requirements in time and labor, and is generally not suitable for monitoring the progress of crystallization since it does not provide results rapidly enough to permit satisfactory control of the process variables.
Typically, methodologies for determining the appropriate reaction period are directed more to maximizing product yield, rather than to achieving other properties such as a particular crystal size. Further, the maximum product yield is often reached long before the reaction period is terminated, thereby unnecessarily consuming time and resources, and sometimes yielding unwanted by-product phases. Batch-to-batch process variations may also yield inconsistent physical properties between batches. In the absence of a reliable method for determining the endpoint of the synthesis, the reaction mixture may be heated for an unnecessarily prolonged period of time with the concomitant production of undesirably large crystals. It would be desirable to have a method for monitoring the progress of the crystallization, for batch and continuous molecular sieve synthesis processes that could provide information on crystallization more rapidly than conventional powder XRD analysis.
It would also be desirable to monitor the progress of molecular sieve crystallization such that the endpoint of the synthesis reaction could be predicted in advance of the actual endpoint. The early determination of the reaction endpoint would allow the synthesis process to be stopped at a time when the molecular sieve crystals are at their most desirable, for example, with respect to crystal size.
Therefore, there is a need for methods for monitoring crystallization during molecular sieve synthesis that allow the early detection of the endpoint of the reaction, thereby enabling the consistent and reliable production of molecular sieves having desirable characteristics, in the minimum amount of time and with maximum energy efficiency.